Analog-to-digital (A/D) converters are typically utilized to convert analog voltages to digital voltages, i.e., digital values representative of analog voltages. Conventionally, high and low reference voltages applied to the inputs of an A/D converter determine a range within which an applied analog voltage can fluctuate. The A/D converter then converts the analog voltage to the closest of several digital voltage "steps", which are determined by the number of bits and the range of the A/D converter. The number of steps within a specified range are given by the formula resolution=2.sup.(# bits), and the step size of the A/ D converter is given by the formula ##EQU1##
By way of example, a 4-bit A/D converter having a range of 8 volts has 16 steps (24 steps) representative of 0.5 volts each (8/2.sup.4 volts/step). When an analog voltage is applied, the A/D converter converts the analog voltage to the closest 0.5 volt step. If, for instance, an analog voltage of 7.44 volts is applied to the A/D converter, a digital output representative of 7.5 volts results. If the digital output is provided in a binary number form, for example, the output would be 1-1-1-0.
In radio communication applications, conventional A/D converters of the type described above are utilized to convert received radio signal voltages from analog format to digital format for subsequent generation of digital data. When the incoming radio signal is relatively large in amplitude, i.e., the voltage levels of the signal vary widely, the digital transformation performed by the A/D converter may be quite useful for accurate generation of digital data. However, when the signal amplitude is small, the results of the A/D conversion may not be quite so useful. Specifically, when the voltage levels of an incoming signal vary over only a few of the A/D steps, the resolution of the A/D converter may be such that no distinction in the digital output can be made.
If, for example, a 5.3 volt signal which fluctuates from 5.00 to 5.60 volts is received by the above-described 8-V, 4-bit A/D converter, only two of the 0.5 volt A/D steps are encompassed. As a result, the digital output will vary only from 5.0 to 5.5 volts as the signal fluctuates, and the smaller voltage variations cannot be distinguished.
One solution to this problem is to utilize an A/D converter having a smaller voltage range and, therefore, smaller A/D steps. However, there is then a risk that the incoming radio signal might be entirely out of the range of the A/D converter if the voltage level of the signal varies. In this situation, the radio communication device might never detect the presence of the incoming signal.
Another solution to the problem is to utilize an A/D converter having an increased number of bits, which results in a larger number of A/D steps over the same voltage range. The digital output of the A/D converter may then more accurately distinguish between small voltage variations in the analog radio signal. However, this increased resolution can be obtained only at a greater cost, which is typically reflected in the price of the final product, i.e., the radio communication device, utilizing the A/D converter. Furthermore, increasing the resolution can sometimes affect the speed of the A/D converter. Therefore, the use of A/D converters having greatly increased resolutions may not be feasible for applications in which only a small minority of the incoming radio signals have very small voltage variations or for applications in which conversion speed is of concern.
A need exists, therefore, for an A/D converter which yields accurate digital voltages for applied analog radio signals having small voltage variations. Additionally, the A/D converter should be able to process analog radio signals received across a large voltage range without a significant reduction in conversion speed.